Difficulty- Not difficult at all. Working with the hog clips can be a *****, especially if they are leather seats. All you really need is patience, some needle nose pliers, and some basic wiring skills.
Cost- I think the heat element alone cost me 150 on ebay. I bought the element and m3 seats to put in my old mx-3 but they were way too large so i took the element and put it in the existing seat.
Where to buy- Ebay. Ebay. Ebay